The ARENA (Advanced Real-Time Earth monitoring Network in the Area) project has been proposed by JAMSTEC(1)2)). The data transmission system for seafloor monitoring cable network has been discussed in the transmission sub-workgroup of ARENA Project. The paper reports the results of discussions in the sub-workgroup. In the ARENA Project, the total length of cable is more than 1000km, and observation nodes are set at about 50km intervals, with various sensors installed in these nodes. The objective is to construct the data transmission system from the sensors in the seafloor to the landing stations. Various conditions should be considered such as transmission range, time accuracy, extendibility, cost, affinity with IP, power consumption.